The invention relates to a high strength alpha-beta alloy having an improved combination of strength, machinability and ballistic properties.
Titanium base alloys are used in applications requiring high strength-to-weight ratios, along with elevated temperature properties and corrosion resistance. These alloys may be characterized as alpha phase alloys, beta phase alloys, or alpha-beta alloys. The alpha-beta alloys contain one or more alpha stabilizing elements and one or more beta stabilizing elements. These alloys can be strengthened by heat treatment or thermo-mechanical processing. Specifically, the alloys may be strengthened by rapid cooling from a high temperature in the alpha-beta range or above the beta transus temperature. This procedure, known as solution treatment, is followed by an intermediate-temperature treatment, termed aging, to result in a desired mixture of alpha and transformed beta phases as the principle phases in the microstructure of the alloy.
It is desirable to use these alloys in applications requiring a combination of high strength, good machinability and ballistic properties.
It is accordingly an object of the present invention to provide an alpha-beta titanium-based alloy having this desired combination of properties.
Alpha-beta titanium alloy, comprising:
Al: 4.5 to 5.5 wt %
V: 3.0 to 5.0 wt % (preferably 3.7 to 4.7 wt %)
Mo: 0.3 to 1.8 wt %
Fe: 0.2 to 1.2 wt % (preferably 0.2 to 0.8 wt %)
O: 0.12 to 0.25 wt % (preferably 0.15 to 0.22 wt %)
Balance titanium and incidental elements and impurities with each being less than 0.1 wt % and 0.5 wt % total.
The alloys in accordance with the invention have aluminum as an essential element within the composition limits of the invention. If aluminum is lower than 4.5%, sufficient strength will not be obtained. Likewise, if aluminum is higher than 5.5%, machinability will be inferior.
Vanadium is an essential element as a beta stabilizer in the alpha-beta titanium alloys in accordance with the invention. If vanadium is less than 3.0%, sufficient strength will not be obtained. Likewise, if vanadium is higher than 5.0%, the beta-stabilizer content of the alloy will be too high resulting in degradation of machinability.
Iron is present as an effective and less expensive beta stabilizing element. Normally, approximately 0.1% iron results from the sponge titanium and other recycle materials used in the production of the alloy in accordance with the invention. Otherwise, iron may be added as steel or as ferro-molybdenum master alloy since the alloy of the invention has molybdenum as an essential element. If iron is higher than about 1.2%, machinability will be adversely affected.
Molybdenum is an effective element to stabilize the beta phase, as well as providing for grain refinement of the microstructure. If molybdenum is less than 0.3%, its desired effects will not be obtained. Likewise, if molybdenum is higher than 1.8%, machinability will be degraded.
Oxygen is a strengthening element in titanium and its alloys. If oxygen is lower than 0.12%, sufficient strength will not be obtained, and if oxygen is higher than 0.25%, brittleness will occur and machinability will be deteriorated.